In cell-based integrated circuit design, a library of predefined physical structures, known as cells, is provided. Each cell performs a particular function such as NOR, NAND, and INV, for example. The physical structures typically include one or more transistors that are defined in base layers of the integrated circuit and are interconnected by one or more metal or via layers. During circuit design, a node level description of the circuit, known as a netlist, is produced that describes how the inputs and outputs of the cells are to be connected in order to perform one or more higher-level functions. Using the netlist, a placement tool selects the best position for each cell in a layout area. A routing tool then adds signal connect lines to connect the inputs and the outputs of the cells to each other according to the netlist. The signal connect lines are connected to the cells through vias in the cells that are referred to as input or output pins. Some cells have multiple vias that can be used as pin locations during routing. The routing tool selects the best pin locations for the circuit as a whole based on considerations such as parasitic capacitance and timing.
Electromigration (EM) is the transport of metal atoms when an electric current flows through a metallic structure in an integrated circuit (IC). For instance, EM can cause metal atoms to be removed from a portion of a metal trace thereby creating a void and possibly an open-circuit failure in the integrated circuit.
Traditionally, EM has been a significant concern in power delivery networks, where the direction of current flow is generally unidirectional, resulting in a steady migration pattern over time. Of late, two new issues have emerged. First, it has become increasingly important to consider the effects of electromigration in signal wires, where the direction of current flow is bidirectional (for this reason, this is often referred to as AC EM), due to increased current densities and Joule heating effects that accelerate EM. Second, traditional EM analysis has focused on higher metal layers that connect the cells together. However, with shrinking wire dimensions and increasing currents, the current densities in lower metal layers within the cells are also now in the range where EM effects are visible. EM effects are visible at effective current densities of about 1 MA/cm2.